Method for diffing large 834 enrollment files without crashing Excel

Working with enrollment files that run to hundreds of megabytes can freeze Notepad++ or cause Excel to truncate data, which makes routine month-over-month comparisons hard. The article proposes a member-level approach: split each file into member records at INS segments, build a stable key per member, and store a hash and file offset for each. Streaming the new file against that index classifies members as added, unchanged, changed, or possibly termed while keeping memory tied to member count rather than file size. It also warns that full and change 834 files must be treated differently, since an absent member means termination in one but not the other. A small free command-line tool, x12diff, is presented as an implementation that detects the file type from BGN08.